This invention relates to the field of heat sealing. More specifically, it relates to the field of heat sealing plastic sheets such as polyvinyl chloride and like materials along their ends. This is sometimes referred to as an overlap seal by which the end of one sheet is joined to the end of a second sheet in order to form various products.
In the prior art it is known to seal two sheets of flexible plastic material by the use of sealing dies which apply pressure and radio frequency energy. The pressure on the sheets produces intimate surface contact while the radio frequency energy effects dielectric heat sealing in a manner well known in the art.
A particular problem with this method of heat sealing sheets of flexible plastic is the weakness of the sheets in the region of the seal. Often in the prior art the sealing process causes the seal or the areas adjacent the seal to become thinner than the gauge of a single sheet of plastic in spite of the fact that two sheets are overlapped in this area. This may be due to the extrusion of the heated material away from the area of the seal due to the application of pressure by the dies. As indicated in FIG. 2 of the drawings, this extrusion of material forms between the sheets which are being sealed together.
In certain applications it is necessary that the seals between sheets of plastic be as strong as or stronger than the material itself.
It is accordingly an object of the present invention to provide a method of sealing two sheets of flexible plastic together in a manner which avoids extrusion of material between the sheets adjacent the sealed area.
It is another object of the present invention to provide an improved sealing die which can produce a seal which is as strong as or stronger than a single sheet of the material being sealed.
It is a further object of the present invention to provide a means for dielectrically sealing at least two sheets of flexible plastic material together with negligible extrusion of material between the sheets.
Other objects and advantages of the invention will be apparent from the remaining portion of the specification.